1. Field of the Invention
Multi-element optical systems continue to find utility in a variety of applications. By constructing the optical system from a plurality of individual optical elements, it is possible to achieve a higher level of performance (sharpness of image, freedom from distortion, brightness, resolving power, and the like), since even when individual relatively inexpensive elements are utilized, aberrations introduced by one element of the system may be compensated for by similar but opposite aberrations occurring elsewhere in the system.
In more recent times, the designing of such multi-element systems (and in particular, of especially complex systems using off-axis aspheric elements) has been greatly facilitated by the use of the digital computer which may be programmed to simulate the performance characteristics of each of the individual elements of the system, and which is then able to calculate the overall system performance by tracing a number of different simulated ray paths from one end of the system to the other, performing the same calculations over and over again as individual parameters (such as the focal length or location or index of refraction) of each particular element are varied. In such a manner, the optical designer can be sure that within the constraints specified by a particular design concept, he has optimized the variables at his disposal in accordance with the requirements of a particular application.
Another recent advance in optics technology relates to the precision manufacture of rotationally symmetric optical elements (such as mirrors in the shape of paraboloids and hyperboloids and other solid conic curves) by precision numerically controlled machining on an airbearing lathe using a polished diamond cutting tool. Especially at the longer wavelengths employed in infrared viewers, the resultant machined surface is sufficiently free from significant surface irregularities that no further polishing is required.
Thus, it can be seen that as a result of state of the art design and manufacture methods, it is possible to create an optical system having a theoretically optimal performance from individual elements that may be readily and inexpensively fabricated. However, in order to secure the optimal performance inherent in the system design, it is necessary that the individual elements be precisely aligned--both rotationally and translationally--with respect to each other.
2. Description of the Prior Art
Traditionally, this has been achieved by adding the individual elements to the system under construction one at a time as accurately as possible, testing the performance of the resultant subsystem at every step of the construction process by means of precision optical measurement techniques, making minor manual adjustments to the position of the element introduced at the current step until a pre-established criterion unique to that step has been met. As can be seen, this is essentially a manual process that requires a highly skilled optical technician, since each successive step of the process requires that all prior steps had been done within the pre-allocated error budget. In the event that the completed system did not perform satisfactorily, it was not always possible to determine which step (or steps) had not been performed with the required accuracy. If the misalignment was relatively minor and did not involve several mutually interdependent misadjustments, the following approach would sometimes prove effective. Each possible adjustment in the entire system was tested in sequence to determine whether or not a minor perturbation in its setting would effect a noticeable improvement in the performance of the system as a whole. If no noticeable improvement resulted, the adjustment was left as it was; otherwise the amount of the adjustment would be increased (or decreased) until no further improvement in overall system performance could be observed. In other words, the technician aligning the system followed up the slope of the curve expressing performance as a function of the adjustment variable in question until a plateau was reached. However, as should be apparent, there was no guarantee that the plateau would be a true maximum or that the alignment in question was sufficiently independent of the other adjustments in the system that such a sequential method would necessarily result in an optimal or even an acceptable level of performance being attained. In the event that such a sequential alignment attempt proved ineffective, then the system had to be completely dismantled and the whole assembly procedure repeated. There was no prior art technique of general applicability that could be used to determine which combination of adjustments was required in order to optimize the performance of the system as a whole.
Accordingly, although there have been substantial advances such that it is now possible to fabricate optical elements of high quality at low cost, there has not been a corresponding advance in the art of assembling those individual elements in accordance with a particular system design.
The problem discussed above may be referred to as the "initial alignment" problem and is primarily the concern of the manufacturer.
However, there is a related problem that is primarily the concern of the user, that is to say the problem of "maintaining alignment" in the system when it is subjected to adverse environmental conditions and even possible physical abuse. In the past, it was considered the responsibility of the designer and manufacturer of the optical system to mount the individual elements within a mechanical frame that was sufficiently sturdy and rigid (and if necessary provided with thermal compensation mechanisms which maintain critical a priori relationships even when thermal forces have resulted in the expansion or contraction of the individual optical and/or mechanical elements). Obviously, the practical difficulties in maintaining alignment by means of prior art techniques were less severe for those applications in which the size of the individual optical elements as well as the size of the overall system was not especially large and wherein the environmental conditions were not especially extreme (such as a telescopic sight on a hunting rifle). On the other hand, as the size and weight of the system and its individual elements is increased and as the anticipated environmental conditions become more extreme, a passive mechanical solution becomes increasingly cumbersome, expensive and impractical.
As an example of an optical system wherein it clearly would be impractical to rely solely on mechanical design expedients to maintain the system in alignment, mention may be made of a high resolution telescope intended to be used aboard a space shuttle vehicle. When pointed towards the sun, such a telescope will experience a high influx of heat. On the other hand, when it is pointed to outer space, heat will actually radiate from the telescope into the depths of space. Furthermore, if it is to produce a bright image over a wide field of view, a large effective aperture is required which means larger (and more massive) individual optical elements that must be aligned even more accurately. Finally, even though the system will be subjected to varying gravitational conditions and high vibration and other mechanical stresses (particularly during the launch of the space vehicle), for reasons of cost it is obviously important to keep the overall mass of the system as low as possible. Moreover, in addition to the original alignment of the system during its manufacture on the ground there is the re-occurring need for maintaining (or even re-establishing) the alignment of the individual elements while the system is in orbit. As can be appreciated, the prior art alignment techniques were trial and error in nature and depended on the intuition and experience of a trained technician. However, even if they could readily be adapted to the automated environment of an active alignment system, because of their sequential nature, they would still be time-consuming and would therefore detract from the efficient use of the system and would in any event have a rather lengthy time constant that would not be adequate in an especially adverse environment.